qm1usn

From my own experience with the US41, aside from the possibility of not enough air exit space, you may be too lean. If the exit opening is sufficient, try opening your hi needle an 1/8 of a turn. If it improves, try another 1/8 turn. Mine acted like that until I richened it up and ran a couple of gallons through it. Once set up, these are good running and user friendly.

Phil